An amount of average data used by mobile communication users has been geometrically increased with use of a mobile station such as a smart phone. In addition to that, users' demands for a higher data transmission rate have been continuously increased. A method of providing a generally high data transmission rate includes a method of providing communication using a wider frequency band and a method of increasing frequency usage efficiency. However, it is very difficult to provide the higher average data transmission rate through the later method. This is because communication technologies of a current generation provide frequency usage efficiency close to a theoretical limit and thus, it is very difficult to increase the frequency usage efficiency up to that or more through a technical improvement. Accordingly, it can be said that a feasible method for increasing the data transmission rate is a method of providing data services through the wider frequency band. At this time, the thing to consider is an available frequency band. In view of the current frequency distribution policy, a band in which a broadband communication of 1 GHz or more is possible is limited and a practically selectable frequency band is only the millimeter wave band of 30 GHz or more. Such a signal of the high frequency band causes severe signal attenuation according to a distance differently from a signal of a frequency band of 2 GHz used by the conventional cellular systems. Due to such signal attenuation, service providing coverage of a base station using the same power as the conventional cellular systems will be considerably reduced. In order to solve this problem, a beam forming technique is widely used which concentrates transmission/reception power into a narrow space to increase transmission/reception efficiency of an antenna.
FIG. 1 illustrates a base station and a mobile station for providing beam forming by using an array antenna.
Referring to FIG. 1, each of cells Cell-0, Cell-1, and Cell-2 of the base station 110 includes a plurality of array antennas Array 0 and Array 1. The base station 110 may transmit data, while changing a direction of a downlink transmission beam Tx by using the array antennas Array 0 and Array 1. In addition, a mobile station 130 may also receive data while changing a direction of a reception beam Rx.
In a system for performing communication by using the beam forming technique, the base station 110 and the mobile station 130 provide data services by selecting a direction of transmission beams and a direction of reception beams showing an optimal channel environment among various directions of the transmission beams and the reception beams. Such a process is identically applied to an uplink channel for transmitting data from the mobile station 130 to the base station 110 as well as a downlink channel for transmitting data from the base station 110 to the mobile station 130.
When the number of directions of a transmissible beam of the base station 110 is N and the number of directions of a receivable beam of the mobile station 130 is M, an optimal downlink transmission/reception direction is selected through the following simplest method. The base station 110 transmits a previously promised signal at least M times or more in each of the N transmissible directions, and the mobile station 130 receives the N transmission beams by using M reception beams. According to such a method, the base station 110 should transmit a specific reference signal at least N×M times, and the mobile station 130 should receive the reference signal N×M times to measure reception strength of the received signal. The mobile station 130 may determine the direction corresponding to the measurement value with the highest reception strength among the N×M measurement values as an optimal transmission/reception beam direction, namely, a combination of an optimal transmission beam direction and an optimal reception beam direction. The process of transmitting a signal one or more times in every transmittable direction by the base station 110 as described above is referred to as a beam sweeping process, and the process of selecting the optimal transmission/reception beam direction by the mobile station 130 is referred to as a beam selection process. The process of selecting the optimal downlink transmission/reception beam may also be identically applied to an uplink transmission/reception process of transmitting data from the mobile station 130 to the base station 110.
FIG. 2 illustrates a signal transmission scheme of a base station 110 in a beam forming system.
Referring to FIG. 2, the base station 110 is installed at a location of a specific height 201 and has a predetermined beam width 202. The beam width of the base station may be defined for each of an elevation angle and an azimuth. Further, a transmission beam of the base station 110 transmitted in a direction corresponding to a specific elevation angle 203 is illustrated in FIG. 2. Since the base station 110 is generally located at a location higher than the mobile station 130, the term “declination angle” may also be used instead of the elevation angle. However, the term “elevation angle” will be used hereinafter. The azimuth is omitted in FIG. 2
FIG. 3 illustrates combinations of elevation angles and azimuths of a transmission beam transmitted from a base station 110. The base station 110 is installed in such a way as illustrated in FIG. 2. The base station 110 is installed at a height of 35 m. The base station 110 transmits a transmission beam having a beam width of 5 degrees for each of the elevation angle and the azimuth. The base station 110 transmits such a transmission beam within one sector having coverage of an angle of 30 degrees and a distance of 200 meters. Each of mobile stations may use four reception beams RX1, RX2, RX3, and RX4. FIG. 3 illustrates a case in which the base station 110 configures one sector having the coverage of the angle of 30 degrees and the distance of 200 meters by using ninety six transmission beams having the beam width of 5 degrees for each of the elevation angle and the azimuth.
The transmission beams transmitted by the base station 110 are spread and transmitted in a fan shape when there is no obstacle. However, for convenience of description, each of the transmission beams reaches the ground, with a rectangular shape in the embodiment illustrated in FIG. 3. The rectangles illustrated in FIG. 3 represent ninety six areas where the transmission beam with a specific elevation angle and a specific azimuth reaches the ground. The ninety six transmission beams are transmitted to a more remote area with the elevation angle increased, and the transmission beam transmitted far from the base station is received in a larger area while receding from the base station. The percentage written in each rectangle in FIG. 3 indicates a ratio of an area occupied by a corresponding region in which a transmission beam transmitted to a predetermined position is received, in the entire 96 regions. As illustrated in FIG. 3, it can be seen that the transmission beam transmitted to a boundary area of the base station is received in a very large area as compared with the transmission beam transmitted to an area close to a central region although the transmission beams have the same elevation angle and azimuth. (As illustrated in FIG. 3, assuming a base station height of 35 m and coverage of 200 m, there is a difference of up to 480 times between areas of reception regions.)
In the beam forming system, the mobile station has difficulty in forming a number of transmission/reception beams with a fine beam width similarly to the base station, due to limitations on a physical space, capability, price, and the like. In the embodiment illustrated in FIG. 3, the mobile station 130 forms four reception beams RX1, RX2, RX3, and RX4 to receive the transmission beams transmitted by the base station. In this case, an azimuth beam width of the reception beams is about 90 degrees.
In a case of using transmission beams with a narrow elevation angle and a narrow azimuth as illustrated in FIG. 3, a number of transmission beams and reception areas exist within the base station 110. Particularly, in a case of transmitting a downlink synchronization channel and broadcast control channels, which are transmitted through a sweeping method, by using the narrow transmission beams as illustrated in FIG. 3, repetitive transmission is required one or more times, namely, at least ninety six times toward all narrow transmission beams within the base station 110.
The number of transmissions required for transmitting the downlink synchronization channel and the broadcast control channels through the beam sweeping method is proportion to the number of transmission beams existing within the coverage of the base station 110. Accordingly, the simplest method of reducing transmission overhead of the downlink synchronization channel and the broadcast control channels in the base station 110 as illustrated in FIG. 3 is to support the whole coverage of the base station 110 with a smaller number of transmission beams. For this purpose, the beam width of each transmission beam should be relatively wide.
However, as the beam width increases, a beam forming effect generally decreases in proportion to the beam width. That is, as the beam width decreases, the beam forming effect further increases. When the beam width is decreased for an improvement of the beam forming effect, the number of transmission beams required for supporting one base station area is accordingly increased and thus, the overhead required for transmitting broadcast type channels is increased. As described above, the beam forming effect and the broadcast channel transmission overhead have a trade-off relation.
In order to effectively solve such a problem, a method is generally used for diversifying the beam width used for transmitting broadcast channels and the beam width used for transmitting user data. For example, a transmission beam with a beam width of 30 degrees may be used as a transmission beam for transmitting the broadcast channels by a sector of 60 degrees, and a transmission beam with a beam width of 10 degrees may be used as a transmission beam for transmitting the user data. In the method of using two or more different beam widths as in the aforementioned example, the transmission beam with a relatively large beam width is referred to as a wide beam or a coarse beam. On the other hand, the transmission beam with a relatively small beam width is referred to as a narrow beam or a fine beam. The similar terms may be used in the same way, even in the case where a method of using two or more different beam widths for reception beams is used.
In a general communication system, a mobile station should measure reception capability of a downlink wireless channel used for receiving data and report the measurement value to a base station prior to data reception in order to receive data from a base station through a downlink. The base station determines a time point of scheduling the mobile station and a data transmission rate suitable for a channel situation of the mobile station, by using the reported reception capability information of the wireless channel. In an uplink through which the mobile station transmits data to the base station, the base station directly measures reception capability of an uplink wireless channel and schedules uplink data transmission based on the measured information.
An operation of transmitting/receiving data in the beam forming system is the same as the transmission/reception operation of the general communication system. However, in the beam forming system, the number of wireless channels (or resources) capable of transmitting/receiving data is increased by the number of transmission/reception beam pairs. Namely, in the beam forming system using the narrow transmission beams as in FIG. 3, the base station 110 may transmit data to the mobile station 130 at a specific location, by selecting one or more transmission beams among the narrow transmission beams that the mobile station 130 can receive. In order to help the base station 110 select a transmission beam, the mobile station 130 receives reference signals transmitted through the respective narrow transmission beams by using each of the reception beams and reports the measured signal strength to the base station.
Such a frequent signal measurement causes excessive power consumption of the mobile station 130. Further, considerable uplink resources should be allocated so that the mobile stations 130 may report the measurement result. Thus, when the report resources are allocated to all the mobile stations 130, a lot of resources are used for transmission of control information, thereby deteriorating system efficiency.